Petri Dishes VS Giant Particle Colliders

I’m glad the Large Hadron Collider is getting so much press; it’s a rare day that physics takes center stage in the minds of everyday people. Searching for fundamental unifying forces of the universe by smashing atomic particles at near-light speed into each other inside the largest machine ever constructed by human beings is pretty damn awesome. The cutting-edge discoveries that come from it (or the ones that conspicuously fail to) will undoubtedly play a large role in our understanding of the most illusive workings of the universe. So you might be surprised to know that I find research that began 20 years ago with some bacteria in a jar to be even more exciting.

Richard Lenski’s research at Michigan State University is breathtaking. The procedure is stupefyingly simple but the results are stunningly elegant. Not only has he observed evolution in a laboratory setting, but he’s provided snapshots of every step along the way. And when a novel ability was eventually coughed up by natural selection, he rebooted from past generations to see exactly where and how it happened.

It all went a little something like this:

In 1988 Lenski and his team started out with a single microbe of E. Coli, allowed it to divide, and from its offspring he started 12 colonies. They were kept in separate containers, each filled with a glucose-citrate solution. Every day, samples from each colony were frozen and the solution replenished. (E. Coli feed on glucose, but it is an identifying attribute of the bacteria that they can not consume citrate because they are unable to pull it through their membranes.) This process provides a record of genetic change every 500 or so generations that can be resurrected and compared to bacteria at any other stage.

From Carl Zimmer’s article A New Step in Evolution:

Over the generations, in fits and starts, the bacteria did indeed evolve into faster breeders. The bacteria in the flasks today breed 75% faster on average than their original ancestor. Lenski and his colleagues have pinpointed some of the genes that have evolved along the way; in some cases, for example, the same gene has changed in almost every line, but it has mutated in a different spot in each case. Lenski and his colleagues have also shown how natural selection has demanded trade-offs from the bacteria; while they grow faster on a meager diet of glucose, they’ve gotten worse at feeding on some other kinds of sugars.

But then… after 33,127 something weird was going on.

The flasks can sometimes get cloudy as a result of foreign citrate-eating bacteria invading accidentally and thriving. When this happened, Lenski and his team would just throw out the contaminated specimen and reboot from the most recent unadulterated version. In one case however, it wasn’t infiltrating alien microbes. The E. Coli had gone through several mutations that allowed it to feast upon the hitherto-unmolested citrate, a feature that has only been seen naturally in E. Coli that has transplanted DNA (in the “wild” E. Coli can sometimes exchange bits of genetic code with other bacteria). These guys evolved citrate-consumption all on their own.

Again from Carl Zimmer:

Blount took on the job of figuring out what happened. He first tried to figure out when it happened. He went back through the ancestral stocks to see if they included any citrate-eaters. For the first 31,000 generations, he could find none. Then, in generation 31,500, they made up 0.5% of the population. Their population rose to 19% in the next 1000 generations, but then they nearly vanished at generation 33,000. But in the next 120 generations or so, the citrate-eaters went berserk, coming to dominate the population.

This rise and fall and rise suggests that the evolution of citrate-eating was not a one-mutation affair. The first mutation (or mutations) allowed the bacteria to eat citrate, but they were outcompeted by some glucose-eating mutants that still had the upper hand. Only after they mutated further did their citrate-eating become a recipe for success.

For more of the technical (read “awesome”) side of things, read the rest of Zimmer’s article, Lenski’s Experimental Evolution website at MSU, and the paper abstract (abstract is jargon for a research paper’s summary and conclusion).

If you’re still wondering to yourself “How on Earth can this guy be more interested in E. Coli evolving than exploding protons generating microscopic black holes that dissipate instantaneously?” then well, you might have a point. I understand how that would be more appealing to some, but when you view this research in the context of all that evolution informs us of it is profoundly exciting.

Every single one of us shares a common ancestor. On top of that, we all share a common ancestor with E. Coli and the only thing that separates us is millions of years of the exact same process that has been observed and documented by Lenski and his colleagues. If that thought doesn’t turn your brain into a mushy pile of awe-goo I don’t know what will.

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3 Responses to “Petri Dishes VS Giant Particle Colliders”

  1. Joel Hicks Says:

    While my brain is not goo-ified per se, this is pretty awesome stuff. Mendel would be proud.

    On another note, have you seen Zeitgeist: The Movie?

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